1. Field of the Invention
The present invention pertains to a process for the production of polyolefin foam beads which have a coarse cell structure and a narrow cell size distribution.
2. Description of the Related Art
A number of processes are known for the production of polyolefin foam beads by dispersion foaming. For example, DE-A-21 55 775 describes a process in which a dispersion of polymer particles in a liquid dispersion medium is heat-treated under pressure and foamed with subsequent decompression. The explanation for the foaming is that the dispersion medium infiltrates cavities of the polymer and acts as a blowing agent on discharging into a low-pressure space. In order to simplify the infiltration, the polymer should contain from 10 to 70% by weight of a filler. This method gives prefoam beads having a fine and uniform cell structure. As the examples show, however, the expansion rate is not uniform. It has been found in practice that conversion of these prefoam beads into foamed moldings is difficult due to the high filler content and the small cell diameter.
EP-A-0 053 333 describes the foaming, in particular, of ethylene-propylene random copolymers using a system comprising polymer particles, water as dispersion medium, a solid dispersion auxiliary and a volatile blowing agent.
EP-A-0 071 981 discloses that polypropylene foam beads can be welded together to provide dimensionally stable moldings, if the size of the gas-filled cells within the foam is adjusted so that not more than 300 cells are present per mm.sup.2 in a cross-section, the molding having a density of from 0.026 to 0.060 g/cm.sup.3 and having a latent heat of crystallization of from 9 to 28 cal/g. However, this publication gives no indication of the technical measures necessary to obtain this cell content.
In addition, EP-A-0 095 109 teaches that, in order to achieve a uniform cell structure and a constant expansion rate during the discharge operation, not only must the internal temperature be kept constant as precisely as possible, but also the internal pressure and the partial pressure of the blowing agent must remain at a constant level.
However, the subsequent introduction of cold inert gases or cold blowing agent, which must evaporate in the reactor due to absorption of heat, depending on its state, means that the reactor contents are cooled, which is undesired. In EP-A-0 290 943, this cooling is avoided by introducing a heating fluid to maintain the liquid level in the reactor, and thus the pressure and temperature, the same. However, since considerable amounts of heating fluid must be introduced, this process uses a significant amount of energy.
It is thus evident from the prior art that considerable effort must be made to keep the temperature of the reactor contents constant during the discharge operation in order to achieve a uniform cell structure of the foam beads.
It is furthermore desired for the processing of the foam beads that they have--in addition to a uniform cell structure and a relatively large cell diameter--an approximately spherical shape so that they fill a mould as completely as possible. This can be achieved by subjecting the polymer particles in the reactor to heat treatment above their crystallite melting point, T.sub.m. In addition to the rounding of the particles to give a spherical shape, this process also provides the benefit that only a relatively small amount of blowing agent is required. However, the resultant foam beads have very small cells and can therefore only be processed with difficulty.
If, by contrast, the heat treatment is carried out in the temperature range below T.sub.m, the foamed particles have the shape of the granules employed, i.e. they are generally more or less cylindrical, which has an unfavorable effect on the surface structure of the moldings produced therefrom.
Neither can the above-described disadvantages be overcome by a combination of the two procedures. If the heat treatment is carried out, for example, at temperatures above T.sub.m and the polymer particles in the reactor are subsequently cooled, before the expansion, to a temperature below T.sub.m, foam beads with cells which are smaller than desired are obtained and which further exhibit considerable shrinkage during conversion into moldings.